It comes down a lot to personal preference but I'm a big fan of the Mede8er stuff. Compares to the Dune and Popcorn Hour players but cheaper and usually more frequent firmware updates compared to most companies. I bought the top of the line MED100X3D (only available through a US site) and it was close to $300. After all the researching I've done I've found the biggest decisions are which chipset you want to go with (Realtek or Sigma), which version of the chipset you want as each have their own issues which limits the media player, and one of the most important ones is how active the company is in putting out firmware to fix what they can or give workarounds when either Realtek or Sigma refuse release new SDK's (containing bugfixes). I guess the last decision is whether you want all the extra stuff like Netflix, Hulu etc, etc. I've found that the more expensive a player gets the less chance you will get the big name stuff like Netflix. The reasoning behind this is that Netflix has specific rules in place concerning what you are allowed to do with the media player and what you aren't allowed to do. As you can imagine quite a few of the companies have given the middle finger to Netflix etc haha.
I have no experience with WD Live or Roku but I can say to stay away from any of the Asus offerings. Poor support and a horrible experience overall.
Mede8er MED1000X3D, MED600X3D, MED500X2 - The greatest show on earth